ci: build inside bread-ecosystem's shared Arch container

Ports breadpaper onto bread-ecosystem's shared, pinned Arch build
image/script instead of installing toolchain/libraries directly on the
bare self-hosted runner. Follows the pattern proven in breadpad:
ci/build.sh clones bread-ecosystem pinned to the sha in
ci/bread-ecosystem.rev and delegates to its ci/build.sh.

breadpaper needs nothing beyond the shared image's base package set
(only clap/anyhow as deps, no image-processing or GTK crates), so no
ci/deps.txt is added.

Also adds check.yml: clippy + test on feature/**/fix/** pushes, as a
fast pre-release-build signal, matching breadpaper's own build/test
flags (--release --locked, no --workspace since this isn't a
workspace).

ci/bread-ecosystem.rev

@@ -0,0 +1 @@
+147cfbbf96ae4b171027defa1130d2caddb934b1

ci/build.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+# Delegates to bread-ecosystem's shared CI build image/script, pinned to
+# the commit in ci/bread-ecosystem.rev — not `main`. bread-ecosystem's CI
+# files now affect every product's release pipeline, so bumping the pin
+# is a deliberate act instead of silent drift (see the bread-theme test
+# that broke here for exactly that reason, before it was pinned by rev).
+#
+# Usage: ci/build.sh cargo build --release --locked
+set -euo pipefail
+
+ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+REV="$(cat "${ROOT}/ci/bread-ecosystem.rev")"
+
+CACHE_DIR="/tmp/bread-ecosystem-ci-${REV}"
+if [ ! -d "$CACHE_DIR" ]; then
+    rm -rf /tmp/bread-ecosystem-ci-*
+    git clone https://git.breadway.dev/Breadway/bread-ecosystem.git "$CACHE_DIR"
+    git -C "$CACHE_DIR" checkout --quiet "$REV"
+fi
+
+bash "${CACHE_DIR}/ci/build.sh" breadpaper "$ROOT" "$@"